As is well known in the art, optical waveguide fibers have a central core surrounded by a cladding, with the core having a higher index of refraction than the cladding. Such fibers are produced by heating the end of a "blank" (also known as a "preform") and drawing fiber from the blank, the diameter of the fiber being controlled through the draw rate. As in the fiber, the blank has a higher index central core surrounded by a lower index cladding, the cross-sectional size of the blank's core and cladding being, of course, much larger than the fiber's core and cladding, e.g., ten to one hundred times larger.
Since various geometric properties of a fiber, e.g., core/cladding diameter ratio and core/cladding concentricity, are determined by the corresponding geometric properties of the blank from which the fiber is drawn, workers in the art have developed various devices for measuring blank geometry. One widely used device for determining a blank's diameter is that sold by LaserMike, Inc. (Dayton, Ohio) under the LASERMIC trademark. This device operates by transversely illuminating a blank and detecting the outer edge of the blank's shadow using an electronic camera.
U.S. Pat. No. 5,408,309 to Shimada et al. describes the use of transverse illumination to measure core/cladding concentricity and blank ellipticity. The patent discusses determining concentricity and ellipticity by rotating a blank and detecting the location of the edges of the core and the cladding as a function of rotation angle with an electronic camera. Various equations are presented for analyzing the camera recordings depending on whether the blank exhibits ellipticity and a decentered core or just one of these defects. An embodiment employing a laser light source is also described wherein residual stress at the interface between the core and cladding layer is said to provide clear images of those layers.